Flange Parallelism Assistant

API 686-aligned Field Check
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Flange Parallelism Assistant — Free Tour

A read-only preview of the Flange Parallelism Assistant. Explore the layout and inputs. The live API 686-aligned check unlocks with a Standard 1-Day Pass or Pro Monthly Subscription.

Parallelism Calculator
Enter the flange diameter and the measured face-to-face gap difference around the joint.
Free Tour preview — explore the layout and inputs. Running the parallelism check is disabled on this preview. Choose a plan to unlock the live Flange Parallelism Assistant.
Use the flange outside diameter (or the bolt-circle diameter the gap is measured across).
Largest gap minus smallest gap, measured with feeler gauge at 4 points around the face.
Awaiting input
Allowable
—
Measured
—
Margin
—
Enter a diameter and a measured gap difference. The check uses the API 686-aligned criterion of 0.001 in per inch of flange diameter (equivalently 1 mil/in or 0.001 mm/mm).
Criterion (API 686-aligned)
Maximum allowable parallelism error ≈ 0.001 × D, where D is the flange diameter. This is the same as 1 mil per inch in US units or 0.001 mm per mm in SI — a common field-acceptance check for piping flanges connecting to rotating equipment.
Always verify the latest project specification and the current edition of the applicable API standard before final acceptance — project specs may be tighter or differ for specific service.

Visual Reference

What the field measurement is looking for.

EQUAL
Parallel — Acceptable
Gap is uniform around the face. Measured difference ≤ 0.001 × D.
min max ANGULAR
Angular — Not Acceptable
Gap varies across the face. If (max − min) > 0.001 × D, the joint fails the check.
D (diameter) 12 o'clock 3 o'clock 6 o'clock 9 o'clock
Measurement Pattern
Take feeler-gauge readings at 4 points (12/3/6/9). Record max and min — the difference is the parallelism error.

How It Works

Three steps from a feeler-gauge reading to a verified result.

STEP 01
Measure 4 Points

At 12, 3, 6 and 9 o'clock, slide a feeler gauge between the faces and record the gap.

STEP 02
Enter Diameter & Gap

Type the flange diameter and the max − min gap. Switch units freely between inch and mm.

STEP 03
Get Pass / Fail

The tool returns the allowable, measured value, margin and a practical interpretation. Open access — no sign-up.

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